For example, Japanese Patent No. 2,639,144 discloses an apparatus for controlling an exhaust gas return valve which comprises a line-disconnection detecting circuit for use in a stepping motor. The control apparatus discerns a specific level of excitation current flowing through to windings of the stepping motor to detect disconnection in winding based on the discerned level of the excitation current.
FIG. 12 shows an electric circuit diagram of a prior art disconnection detecting circuit 100 used in a control apparatus for an exhaust gas return valve. Disconnection detecting circuit 100 comprises excitation coils 121 to 124 provided in a stepping motor 120 and each having one end connected to a DC power source 130 through a resistor 137 or 138; four transistors 101 to 104 each having a collector terminal connected to the other end of excitation coil 121 to 124; current detecting resistors 105 to 108 each connected to an emitter terminal of transistor 101 to 104; four comparators 109 to 112 each having a non-inverted input terminal (+) connected to a junction between each emitter terminal of transistor 101 to 104 and current detecting resistor 105 to 108, and an inverted input terminal connected between two voltage-dividing resistors 131 and 132; four D-type flip flops (DFF) 125 to 128 each having a D input terminal connected to an output terminal of corresponding comparator 109 to 112; a motor drive circuit 140 for simultaneously supplying drive pulses for a base terminal of each transistor 101 to 104 and a trigger input terminal T of each DFF 125 to 128 through a retardant circuit 133 to 136; an AND gate 141 for receiving outputs from all of DFFs 125 to 128; and a control circuit 142 having an input terminal Pi for receiving an output from AND gate 141 and an output terminal Po for forwarding drive signals to motor drive circuit 140.
In operation, control circuit 142 produces drive signals to motor drive circuit 140 which then provides drive signals of four different phases at 90 degree angular intervals for base terminal of transistors 101 to 104 to turn them on and off to drive stepping motor 120. For example, upon turning transistor 100 on, electric current flows from DC power source 130 through resistor 137, excitation coil 121, collector-emitter terminals of transistor 101 and current detecting resistor 105 to ground to activate stepping motor 120. At the same time, drive signals from motor drive circuit 140 are transmitted to trigger input terminal T of DFF 125 with a constant delay time through retardant circuit 133. Dividing resistors 131 and 132 provide a reference voltage for each inverted input terminal of comparators 109 to 112.
In operating stepping motor 120, output signals from output terminal Po of control circuit 142 cause motor drive circuit 140 to produce drive pulses to base terminal of transistor 101 to turn it on so that electric current flows from DC power source 130 through excitation coil 121, transistor 101 and current detecting resistor 105. Electric current through excitation coil 121 and transistor 101 is detected as a corresponding detection voltage applied on current detecting resistor 105, and comparator 109 compares detection voltage on current detecting resistor 105 with reference voltage divided by resistors 131 and 132. When detection voltage on current detecting resistor 105 reaches a predetermined level, comparator 109 produces the output to make DFF 125 produce the output of high voltage level to AND gate 141. Similar operations are carried out in case motor drive circuit 140 produces drive pulses to each base terminal of remaining transistors 102 to 104, and when detection voltage on current detecting resistors 106 to 108 comes up to the predetermined level, comparators 110 to 112 produce the outputs so as to transmit the outputs of high voltage level from DFF 126 to 128 to AND gate 141.
Upon occurrence of disconnection in excitation coil 123, no electric current flows through excitation coil 123 although motor drive circuit 140 gives base terminal of transistor 103 drive pulse, and therefore, there is no detection voltage on current detecting resistor 107. This causes DFF 127 to produce the output of low voltage level to transmit a disconnection signal to input terminal Pi of control circuit 142 through AND gate 141.
Japanese Patent Disclosure Nos. 2000-175486 and 2-26297 demonstrate stepping motors capable of detecting disconnection in motor coil by detecting an elevated voltage applied on a driving element when any disconnection occurs in motor coil of the bifilar turn arrangement in the stepping motor. FIG. 13 shows a disconnection detecting circuit of such a stepping motor.
As shown in FIG. 13, the disconnection detecting circuit comprises two pairs of motor coils or windings 221 to 224 each pair being wound in the bifilar turn arrangement, field effect transistors (FETs) 201 to 204 each connected in series to DC power source 230 and winding 221 to 224, a common current detecting resistor 205 connected between FETs 201 and 202 and ground, a diode 206 connected in parallel to current detecting resistor 205, a common current detecting resistor 207 connected between FETs 203 and 204 and ground, a diode 208 connected in parallel to current detecting resistor 207, a disconnection detecting circuit 240 connected between each winding 221 to 224 and FET 201 to 204, first and second control circuits 210 and 211 for forwarding drive signals to gate terminal of each FET 201 to 204, an excitation signal generator 212 for applying excitation signals to first and second control circuits 210 and 211, and a stop controller 250 for receiving a disconnection signal detected by disconnection detecting circuit 240 to provide stop control signals for first and second control circuits 210 and 211. Disconnection detecting circuit 240 comprises diodes 241 to 244 connected between winding 221 to 224 and FET 201 to 204, a common Zener diode 245 connected to all diodes 241 to 244, resistors 246 and 247 for dividing output from Zener diode 245, and a capacitor 248 connected in parallel to resistor 247. Stop controller 250 has an input terminal connected between resistors 246 and 247.
When no break appears in each winding 221 to 224, stop controller 250 does not produce the output. For example, when winding 221 has a break or cutoff, accumulated electric energy in winding 222 in the bifilar turn arrangement together with winding 221 cannot be released through winding 221, and therefore, high voltage induced in winding 222 causes breakdown of FET 202. Subsequently, electric energy in winding 222 is discharged while electric current flows through a closed circuit including winding 222, FET 202, resistor 205 and DC power source 230. This electric current causes Zener diode 245 to be turned on to provide a trigger signal of high voltage level for stop controller 250. Accordingly, stop controller 250 forwards stop signals to first and second control circuits 210 and 211 to cease operation of FETs 201 to 204.
As mentioned above, prior art disconnection detecting devices require complicated circuit construction and high voltage resistive electric elements to detect disconnection in winding of two phase stepping motor provided with plural windings in the bifilar turn arrangement. Such a circuit construction is expensive in manufacture and also unsuitable to produce an integrated circuit of the disconnection detector. In addition, when two phase stepping motor is operated with unipolar drive, switching elements are on-off switched by a chopper drive of regular current. In this case, if electric energy is accumulated in one winding, and besides, the other winding has a disconnection, counter electromotive force resulted from flyback energy cannot be regenerated through the other disconnected winding, and therefore, an excessive electric potential would be applied to a switching element for excitation of one winding. Thus, there is a large risk that the excessive electric potential may cause damage such as breakdown to the switching element. In this way, any of prior art techniques cannot detect disconnection in winding until the timing to excite the disconnected winding.
An object of the present invention is to provide a device having a simplified circuit structure for detecting disconnection of a stepping motor. Another object of the present invention is to provide a device for detecting disconnection of stepping motor and capable of easily producing the device into an integrated circuit.